The present invention relates to a technique for demodulating data signals recorded on, for example, magnetic cards, and more specifically relates to a highly reliable data demodulation technique for binary data recorded by a pulse code modulating method in which data is recorded by a combination of two types of pulses, F and 2F.
General recording and reproducing devices such as magnetic card readers use a modulation method by which binary data signal are identified by one of two types of pulses, F and 2F. Binary data recorded by this modulation technique is reproduced in the following manner.
A magnetic head and a magnetic stripe on a magnetic card are moved relative to one another to reproduce the magnetically recorded data in the form of an analog signal. Based on the signal waveform of the analog signal, the binary data is determined. FIG. 10 illustrates a general functional block diagram of a conventional data demodulation of this kind and FIG. 11 shows a signal waveform for each block. The recorded signal reproduction at FIG. 11a illustrates the pulse code modulation which under the Japanese Industrial Standard (JIS) is labeled: F and 2F frequency modulation.
In FIG. 10, an output signal of the magnetic head 11, which is obtained when a magnetic card 10 moves relative to the magnetic head, is amplified by two amplifiers 12 and 15. An output signal of the amplifier 12 is supplied to a peak detecting circuit 13 for peak detection, and a peak detection signal of the peak detecting circuit 13 is compared to zero level by a comparator 14 to detect zero crossing points thereof. An output signal of the other amplifier 15 is compared to zero level by a comparator 16 to detect zero crossing points thereof, and its output is input to a timing generation circuit 17. The timing generation circuit 17 changes the output level of the comparator 16 according to the level of the output signal of the comparator 16 which is observed at changing positions of the output signal of the comparator 14. The output signal of the timing generation circuit 17 is input to a data discriminating circuit or CPU 18 for a predetermined signal process to identify the character.
A magnetic stripe of a general magnetic card has not only a significant data region in which a recorded data is substantially stored, but also a sync bit region that comes before the significant data region, an STX code region that indicates the beginning of the recorded data, an ETX code region that comes after the significant data region and indicates the end of the data, LRC code region, and a sync bit region.
The operation of the functional block diagram illustrated in FIG. 10 will be described more specifically referring to FIG. 11 as well. FIG. 11(a) illustrates an example of a signal recorded on the magnetic card 10. The recorded signal is a binary data signal composed of a combination of two kinds of frequencies, F and 2F, and expresses the bit by xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d according to the existence of inversion of signal polarity within a time interval (distance) T equal to one bit. The example of FIG. 11(a) expresses xe2x80x9c01101xe2x80x9d. FIG. 11(b) shows an example of the recorded signal of FIG. 11(a) which is read by the magnetic head 11 and amplified by the amplifiers 12 and 15. The output frequency of the amplifier 12 and 15 which corresponds to the recorded signal xe2x80x9c1xe2x80x9d is twice as long as that which corresponds to the recorded signal xe2x80x9c0xe2x80x9d.
The peak detecting circuit 13 is composed of a differentiating circuit. Therefore, the peak detection output provides a signal waveform in which the zero crossing points appear at the peak positions of the output signal of the amplifier 12, as illustrated in FIG. 11(c). This signal is compared to zero level by the comparator 14 and converted to a digital signal which inverts at the zero crossing positions in the peak detection waveform as illustrated in FIG. 11(d). The output waveform of the amplifier 15 is compared to zero level by the comparator 16 and converted to a digital signal which inverts at the zero crossing positions thereof, as illustrated in FIG. 11(e). The timing generation circuit 17 outputs the signal as illustrated in FIG. 11(f). In other words, the timing generation circuit 17 changes the output level of the comparator 16 according to the level of the output signal of the comparator 16 which is observed the comparator 16 at changing positions of the output signal of the comparator 14. The signal illustrated in FIG. 11(f) is the same digital signal expressing xe2x80x9c01101xe2x80x9d as the signal of FIG. 11(a). Thus, it is understood that the data signal recorded on the magnetic card is demodulated.
The above mentioned performance of reading data recorded on magnetic cards is affected by the condition of card, contamination and wear of the magnetic head, electric noise or mechanical noise from a motor, etc. In other words, a recording medium such as magnetic cards receives various stresses over repetitive use; as a result, the contamination or scratches on the recording medium may cause signals that originally did not exist. Also, basic information once written on the recording medium will not be overwritten even with repetitive use; over the time that the recording medium makes repeated contacts with the magnetic head, the magnetic force decreases, and therefore signal intensity necessary for reproduction becomes insufficient, degrading accuracy of data reading. Further, the resolution power of the magnetic head is decreased due to wear on the magnetic head, causing peak shift.
If error occurs in reading data as above, the performance of reading data recorded on the medium may be degraded, affecting correct data identification. FIG. 4 illustrates an example of a false reading caused by peak shift. In FIG. 4, the correct binary data constituting one character is xe2x80x9c1011101xe2x80x9d where the second bit within the character time interval is originally xe2x80x9c0xe2x80x9d. However, the length of the second bit within the character time interval, which is currently under decoding, is narrower than the original distance between peaks due to peak shift. Consequently, when this second bit is demodulated by a conventional method illustrated in FIG. 10 and FIG. 11, it is falsely decoded as xe2x80x9c1xe2x80x9d and accordingly the bit line is falsely determined as xe2x80x9c1111111xe2x80x9d. In addition, the boundaries between the bits after the third bit are shifted due to peak shift, affecting the successive character interval (distance) and causing a false reading therein.
FIG. 14 illustrates another example of the waveform that contains peaks which originally do not appear or do not appear at expected positions. In this waveform, only one peak should appear between the second bit and third bit; although the original is xe2x80x9c10011xe2x80x9d, three peaks are generated between the two bits for some reasons. If this waveform is demodulated by a conventional method shown in FIG. 10 and FIG. 11, the bit line will be falsely read as xe2x80x9c11111xe2x80x9d.
In the aforementioned FM modulating method, as illustrated in FIG. 3, a constant reference time xcex1T (where 0xe2x89xa6xcex1xe2x89xa61) is set with respect to a time interval (distance) T for one bit, and xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is allocated to the bit by observing polarity inversion in the read signal within the reference timexcex1T. In other words, if there is no polarity inversion within the reference timexcex1T, the bit is defined as xe2x80x9c0xe2x80x9d by the frequency F; if there is a polarity inversion within the reference timexcex1T, the bit is defined as xe2x80x9c1xe2x80x9d by the frequency 2F. With this, false reading caused by peak shift can be prevented to some extent.
However, like the example of FIG. 3, even if the reference timexcex1T is set and the bit is identified by observing polarity inversion of the read signal within the reference timexcex1T, the aforementioned factors may cause a false reading; even when a false reading occurs in only one bit in the bit line, the false reading affects the successive bits in the bit line, resulting in false identification. In other words, according to the above described conventional data demodulating method, the predetermined reference time is given to each bit to determine the binary data for each bit; therefore, if error occurs in identification in even one bit, the error also affects the successive bits.
An object of the present invention is to provide a reliable data demodulating method, in which, for identifying the binary data of each bit, the character time interval (distance) for one character is segmented by a reasonable method, and the element of the character time interval (distance) for one character is used to greatly reduce false readings.
Another object of the present invention is to provide a data demodulating method in which, even when peaks that originally do not exist appear or do not appear at expected positions, false reading can be reduced to a great extent, making a highly accurate data demodulation possible.
An object of the present invention is to provide a data demodulating method by which the result of bit conversion is observed to ensure reliability thereof to take the next step; thus, false readings can be greatly reduced, providing a highly reliably data demodulation.
Another object of the present invention is to provide a data demodulating method by which, even if the character expressed by the segment could not be positively determined by bit conversion, the candidates of the characters are narrowed down, thus improving data reading performance.